Ivan Derkach

Quantum communication is advancing toward large-scale quantum networks, with quantum key distribution (QKD) serving as a key driving technology. However, seamless interoperability between fiber-based and free-space links remains a major challenge for heterogeneous quantum networks. Here we report, to the best of our knowledge, the first continuous-variable QKD (CV-QKD) system distributing secret keys using both coherent and squeezed states over a hybrid channel composed of a 620m free-space link followed by 2km of optical fiber, corresponding to a total loss of 20 dB. Daylight operation is enabled by intrinsic mode filtering provided by a locally generated local oscillator, eliminating the need for complex spectral or spatial filtering. In addition, we introduce an optimized binning strategy that mitigates free-space transmittance fluctuations, resulting in an average of 45% increase in the secure key rate. These results demonstrate the feasibility of CV-QKD across hybrid optical channels and highlight its potential as a plug-and-play solution for heterogeneous quantum networks integrating fiber and free-space infrastructure.

Quantum key distribution (QKD) stands as the most successful application of quantum information science, providing information-theoretic security for key exchange. While it has evolved from proof-of-concept experiments to commercial products, widespread adoption requires chip-based integration to reduce costs, enable mass production, facilitate miniaturization, and enhance system performance. Here, we demonstrate the first fully photonic-integrated continuous-variable QKD (CVQKD) system operating at a classical telecom symbol rate of 16 GBaud. Our system integrates a silicon photonic transmitter circuit (excluding the laser source) and a 20 GHz photonic-electronic receiver, which features a phase-diverse silicon photonic integrated circuit and custom-designed GaAs pHEMT transimpedance amplifiers. Advanced digital signal processing allows our system to achieve the highest reported secure key rate to date, reaching 0.289 Gb/s and 0.246 Gb/s over a 20 km fiber link in the asymptotic and finite-size regimes, respectively. These results establish a record key rate and represent a critical step toward scalable, cost-effective, and mass-deployable quantum-secure communication using photonic-integrated CVQKD systems.

Quantum key distribution (QKD) is a well-known application of quantum information theory that guarantees information-theoretically secure key exchange. While QKD systems are becoming commercially available, large-scale deployment of next-generation QKD systems requires photonic and electronic devices that are low-cost, small, and easily integrated with existing network infrastructure. Continuous variable (CV) QKD is a promising option for large-scale deployment due to its compatibility with standard telecom technology. Despite this, the secret key rates of CV-QKD systems have been limited to a few megabits per second due to the bandwidth bottleneck of the receiver and the limited symbol rate of the transmitter. Here, we present the first discrete-modulated coherent state CV-QKD system operating at a classical telecom symbol rate of 10 GBaud. This system generates keys at rates exceeding 0.7 Gb/s over a distance of 5 km and 0.3 Gb/s over a distance of 10 km while being secure against collective attacks in both the asymptotic and finite-size regimes. This is made possible by using a high-speed, co-integrated phase-diverse receiver consisting of a silicon photonics optical front-end and a custom-designed integrated transimpedance amplifier. Additionally, well-engineered digital signal processing is used for quantum state preparation and measurement. Our experiment sets a new record for secure quantum communication and paves the way for the next generation of CV-QKD systems.

In recent years, continuous-variable (CV) quantum communication has become a nascent but promising alternative to secure multiple end-users sharing the same telecommunication backbone. Continuous-variable quantum key distribution (CV-QKD) with reverse reconciliation enables natural scalability from point-to-point communication to quantum access networks with passive quantum broadcasting channels. Here, we report two experimental demonstrations on a $1:4$ quantum communication network with protocols advancing from two fronts, either moving beyond the asymptotic limit or the Gaussian assumption of the modulation. We demonstrate for the first time a network of four active users simultaneously achieving high secret key rates with finite-size security against collective attacks. A comprehensive security analysis was performed to cover different scenarios on how each end user prefers to trust others. In addition, with the same optical configuration, we implement an initial CV quantum network accounting for four users (two active) with discrete modulation with the conservative untrusted broadcast protocol. The results establish the viability of CV quantum network for practical security with imperfect devices when broadly deployed to connect end users to existing infrastructures.

Integrating quantum key distribution (QKD) with classical data transmission over the same fiber is crucial for scalable quantum-secured communication. However, noise from classical channels limits QKD distance. We demonstrate the longest-distance continuous-variable QKD (CVQKD) over 120 km (20 dB loss) coexisting with a fully populated coarse wavelength division multiplexing system. Natural mode filtering of the local oscillator and phase noise mitigation enabled this without additional filtering or wavelength reallocation. Benchmarking against a commercial discrete-variable QKD system and considering finite-size effects confirms the feasibility of CVQKD as a plug-and-play solution for typical 80–100 km long-haul optical networks. Our results set a record distance for CVQKD, showing its potential for cost-effective, large-scale deployment in existing network infrastructure.

We report the experimental demonstration of multi-user continuous-variable quantum key distribution based on a passive optical network (QPON) that supports se- cure key generation for 5 users simultaneously. This is achieved considering practical PON topology with an 11 km span of access links.

We address and theoretically analyze continuous-variable quantum key distribution (CV QKD) with noisy squeezed states. The noise in such states unavoidably emerges due to optical losses in the state preparation and has to be taken into account in any practical scenario, with the outcomes depending on the trust assumption on such noise. We show that the untrusted noise should pessimistically be allocated to the anti-squeezed (AS) quadrature and can break the security of the protocols already in the asymptotic regime. In the finite-size regime we analyze the impact of the AS noise on the parameter estimation, showing that it limits the performance of the protocols even if assumed trusted and requires the protocol modifications in terms of modulation and detection schemes. We also consider the noisy squeezed-state CV QKD in the channels with transmittance fluctuations (typical for the atmospheric channels) and show that even trusted AS noise can break the security of the protocols in this regime due to fluctuations-related channel excess noise, which has to be assumed untrusted. Our results demonstrate the importance of the squeezing purity in practical realizations of squeezed-state CV QKD.

Squeezed states of light promise significant advantages for enhancing the performance of continuous-variable quantum key distribution (CV-QKD) systems. These advantages include the ability to reach longer distances, tolerate higher levels of excess noise, and operate at lower information reconciliation efficiency. So far those advantages were only predicted in theory. In this work, we experimentally demonstrate a CV-QKD system over 40 km fibre using squeezed light achieving a secret key rate of 0.0318 bits per channel use, surpassing the equivalent coherent state system. Similar to state-of-the-art coherent state QKD systems our system employs digital signal processing for impairment compensation eliminating the need for complex locking mechanisms and enhancing its suitability for practical implementations.

We develop a novel multi-user protocol and report the first continuous-variable quantum passive optical network (CV-QPON), that supports secure key generation for eight users simultaneously. This is achieved considering practical PON topology with an 11 km span of access links. Depending on the trust assumptions about users we reach 1.5 Mbits/s and 2.1 Mbits/s of total network key generation. Novel CV-QPON protocol exploits the multi-user nature of the network allowing to extend the network size and enhance individual keys, thus offering a pathway toward establishing low-cost, high-rate, and scalable quantum access networks using standard telecom technologies that directly benefits from the existing access network infrastructure.

Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on physical laws. Continuous variable (CV) QKD based on coherent states and coherent detection is a suitable scheme for integration into existing telecom networks. However, thus far, long-distance CV-QKD has only been demonstrated using a highly complex transmitted local oscillator scheme, opening security loopholes for eavesdroppers and limiting its potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100 km fiber channel. This record-breaking distance is enabled by controlling the phase-noise component of excess noise, using a machine-learning framework for carrier recovery and optimizing the modulation variance. We consider the full CV-QKD protocol implementation and demonstrate the generation of keys secure against collective attacks in asymptotic and finite-size regimes. Our results set an essential milestone for CV quantum access networks realization, where a high loss budget is required, and pave the way for large-scale deployment of secure QK.